Bias stability of depolarized interferometric fiber optic gyroscopes (IFOG) is affected by nonlinear optical effects in the sensing loop of the IFOG. For an ideal broadband light source, no nonlinear optical effects should exist due to the statistical properties of the perfectly thermal source.
When a light source is perfectly thermal, the second order degree of coherence, given by g11(2)(τ)=<I(t)I(t+τ)>/<(I)>2, is such that g11(2)(0)=2, (hereinafter referred to as “g11”). Here I(t) is the instantaneous light intensity, and τ is the delay time. The angle bracket represents average over time. Perfect thermal source means: there is no correlation between the many harmonic or wavelength components that make up the light source spectrum.
However, for less than ideal light sources, the light source statistics are not found to be perfectly thermal, leading to bias instability in the IFOG as optical power fluctuates. This instability has been clearly observed in depolarized IFOGs.